Method for hybrid dry-jet gel spinning and fiber produced by that method

ABSTRACT

A method of spinning a polyacrylonitrile PAN-based precursor fiber comprises extruding a spinning solution of ultra-high molecular weight polyacrylonitrile polymer through a multi-filament spinnerette where the solution has a viscosity of between about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) at a dope extrusion temperature of between about 20° C. and about 26° C., producing a fiber having a diameter of between about 4 and about 10 micron, a tensile strength of between about 500 and about 1100 MPa, and an elastic modulus between about 13 and about 18 GPa.

TECHNICAL FIELD

This document relates generally to the field of fiber spinning and, more particularly, to a hybrid dry-jet gel spinning method for producing a polyacrylonitrile-based precursor fiber.

BACKGROUND

Polyacrylonitrile (PAN) copolymer fiber spinning, as a precursor for carbon fiber and products, has been known for over thirty years. Carbon fibers are primarily used in aerospace applications as well as by the boating, sporting and wind industries. More recently carbon fibers have entered in the automobile industry.

U.S. Pat. No. 4,535,027 to Kobashi et al. discloses one possible method of producing high strength polyacrylonitrile fiber. That process utilizes a spinning solution composed of polymer with a molecular weight of about 2,280,000 daltons at a 5 wt % concentration, with a viscosity within the range of from 500-1,000 Pa-sec at a temperature of 30° C. In order to reduce the viscosity of that spinning solution to allow proper flow characteristics for spinning, the spinning solution is maintained at a temperature of 80° C. at the time of extrusion and dry-jet spun into a bath of 15 wt % sodium thiocyanate concentration at 5° C. and then subjected to drawing without further washing of the filaments for removal of residual solvent. Utilizing a total draw down ratio of 28.8, Kobashi et al. produces a fiber 50 to 300 micron in diameter with a tensile strength of 25.1 g/den, or 100 to 580 MPa, based on the aforementioned fiber diameters. Fiber diameter is known to influence fiber tensile strength, according to Griffith Theory, which details that in the limit of zero volume, there would be no defect and the theoretical strength would be found. Therefore, the larger diameter fiber present in the Kobashi et al. method possesses a higher volume capable of containing a high number of voids. Such voids reduce the strength and quality of the resulting fiber product.

This document relates to a new hybrid dry-jet gel spinning method that utilizes both a temperature induced phase transition used in gel spinning, as well as diffusion used in traditional dry-jet solution spinning to produce a high molecular weight polyacrylonitrile-based precursor fiber with high tensile strength and elastic modulus, small diameter, and, as a result of that small diameter, a fiber that is believed to contain fewer voids than such fiber made by the prior art Kobashi et al. process. The current method can be described as a hybrid of gel spinning and dry-jet spinning due to: the solution (not yet gelled) being extruded through an air gap (dry-jet spinning), the low temperature coagulation to form a gel and impart mechanical integrity to the filaments (gel spinning), and finally a solution spinning solvent exchange (diffusion) during washing (dry-jet spinning), followed by stretching and drying. Such a fiber is characterized by increased strength and improved quality. Accordingly, the hybrid dry-jet/gel spinning method disclosed in the present document represents a significant advance in the art.

SUMMARY

In accordance with the purposes described herein, a method is provided for spinning a polyacrylonitrile PAN-based precursor fiber. The method comprises extruding a spinning solution of ultra-high molecular weight polyacrylonitrile copolymer through a multi-filament spinnerette to produce a PAN-based precursor fiber in the form of an unwindable, multifilament, continuous tow. The polyacrylonitrile-based polymer has a molecular weight of at least 1,000,000 daltons and the solution has a viscosity of between about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) at a dope extrusion temperature of between about 20° C. and about 26° C. In addition the method includes the step of maintaining an air gap between the spinnerette and a coagulation bath of between about 3 and about 10 mm. Further the method includes the steps of stretching the PAN-based fiber in the air gap at a draw down ratio of between about 2.5 and about 8.5, maintaining the coagulation bath between about 0° C. and about 5° C. and further stretching the PAN-based fiber in a series of stretch baths to produce a precursor PAN-based fiber having a diameter between about 4 and about 10 microns, a tensile strength between about 500 and about 1100 MPa, and an elastic modulus between about 13 and about 18 GPa.

In accordance with additional aspects, the method includes using a spinning solution including about 6 wt % polyacrylonitrile copolymer and a coagulation bath consisting of 60 wt % solution of solvent to deionized water. In one embodiment the solvent utilized is N,N-dimethylacetamide. Further the method includes subjecting the PAN-based fiber to a total draw down ratio of between about 15 and about 55.

In one useful embodiment the polyacrylonitrile-based polymer has a molecular weight of between 1,000,000 and 2,000,000 daltons. In one useful embodiment the method includes maintaining a dope extrusion temperature of between about 22° C. to about 24° C. In one useful embodiment the method includes extruding a spinning solution having a viscosity of between about 200 and about 250 Pa-sec. In one useful embodiment the method includes stretching the PAN-based fiber in the air gap at a draw down ratio of between about 4.0 and about 8.5. In one useful embodiment the method includes maintaining the coagulation bath at a temperature of between 0° C. and about 3° C. In one useful embodiment the method includes producing a precursor PAN-based fiber having a diameter of between about 5 and about 6 microns, a tensile strength of between about 800 and about 1100 MPa and an elastic modulus of between about 15 and about 18 GPa. In one useful embodiment the method includes subjecting the PAN-based fiber to a total draw down ratio of between about 40 and about 55.

In the following description there are shown and described preferred embodiments of methods for producing polyacrylonitrile PAN-based fiber. As it will be realized, the methods are capable of other different embodiments and their several details are capable of modification in various, obvious aspects. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated herein and forming a part of the specification, illustrate several aspects of the methods and together with the description serve to explain certain principles thereof. In the drawings:

FIG. 1 is a schematical illustration of a continuous in-line apparatus for producing a PAN-based precursor fiber in the form of an unwindable, multifilament, continuous tow;

FIG. 2 is a plot illustrating steady shear viscosity for four different spinning solutions using the present method and two different spinning solutions using the method disclosed in the Kobashi et al. '027 patent;

FIG. 3 is a plot illustrating fiber tensile properties for fibers produced using the current hybrid method and fibers produced using the method disclosed in the Kobashi et al. '027 patent; and

FIG. 4 is a plot illustrating fiber tensile properties for a number of different fibers provided using the current hybrid method versus a fiber (a) produced using the Kobashi et al. method from the '027 patent and (b) a commercially produced precursor PAN fiber, according to Matsuhisa et al. U.S. Pat. No. 6,428,892 B2 (Toray Industries, Inc.).

Reference will now be made in detail to the present preferred embodiments of the method, an example of which are illustrated in the accompanying drawings.

DETAILED DESCRIPTION

Reference is now made to FIG. 1 schematically illustrating an apparatus 10 for completing the new and improved hybrid dry-jet gel method of spinning a polyacrylonitrile PAN-based fiber with a draw down ratio as high as 55 for production of a fiber with small diameter and high tensile strength. The apparatus 10 includes a reservoir 12 for holding a spinning solution 14. That spinning solution includes between about 3 and about 8 wt % of polyacrylonitrile copolymer and most typically about 6 wt % of polyacrylonitrile copolymer in an appropriate solvent. Appropriate solvents include but are not limited to N,N-dimethylacetamide, dimethylsulfoxide, dimethylformamide, sodium thiocyanate, other similar solvents, and mixtures thereof. The polyacrylonitrile-based polymer has a molecular weight of at least 1,000,000 daltons and the spinning solution has a viscosity of between about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) at a dope extrusion temperature of between about 20° C. and about 26° C.

In one embodiment the polyacrylonitrile-based polymer has a molecular weight of between 1,000,000-2,000,000 daltons. In one embodiment the polyacrylonitrile-based polymer has a molecular weight of about 1.5 million daltons. In one embodiment the dope extrusion temperature is maintained between about 22° C. and about 24° C. In one embodiment the spinning solution has a viscosity of between about 200 and about 250 Pa-sec (at a shear rate of 1 l/sec) at a dope extrusion temperature of between 20° C. and 26° C. or between about 22° C. and about 24° C.

Spinning solution 14 is delivered from the reservoir 12 to a spinnerette 16 through which the spinning solution is extruded into individual fibers F in an air gap 17 provided between the spinnerette and the coagulation or plunge bath 18. The air gap 17 between the spinnerette 16 and the coagulation bath 18 has a length of between about 3 and about 10 mm. In one embodiment the PAN-based fiber is stretched in the air gap at a draw down ratio of between about 2.5 and about 8.5. In another embodiment the PAN-based fiber is stretched in the air gap 17 at a drawn down ratio of between about 4.0 and about 8.5.

Typically, the coagulation solution in the coagulation bath 18 comprises between about 40 and about 65 wt % of solvent aqueous solution (e.g. 60 wt % N,N-dimethylacetamide aqueous solution) that is maintained at a temperature of between about 0° C. and about 5° C. In one embodiment the coagulation solution in the bath 18 is maintained at a temperature of between about 0° C. and about 3° C. Fibers F in the coagulation bath 18 are drawn through the godet 20 which includes a series of rollers 21. The fibers F are stretched at a draw down ratio of up to 8.5 in the coagulation bath 18.

The fibers F pass from the godet 20 to a first wash bath 22 filled with, for example, a solvent aqueous solution of, for example, 50 wt % N,N-dimethylacetamide. The fibers from the bath 22 undergo stretching in the bath 22 at a draw down ratio of up to 1.6.

Fibers F from the godet 24 are then passed through up to 4 additional washing/stretching baths 26 filled with a solution of various concentrations of solvent aqueous solution maintained at a temperature of between about 1° C. and about 30° C. before being drawn through the godet 28. The fibers F undergo additional stretching in the baths 26 between the godets 24 and 28 by a draw down ratio of up to 1.6.

Next the fibers F are delivered from the godet 28 to a hot water bath 30 including deionized water maintained at a temperature of about 90° C. The fibers F are then drawn through the heated godet 32 after undergoing stretching at a draw down ratio of up to 2.3 in the hot water bath 30.

As the fibers F pass through the rollers 34 of the heated godet 32 they are dried before entering a hot glycerol bath 42 maintained at a temperature of between about 150° C. and about 180° C. and typically about 170° C. The fibers F are then drawn through the godet 44 before being delivered to a sequence of two hot water baths 46 including deionized water at a temperature of between about 85° C. and about 100° C. for removal of residual glycerol. The fibers F then pass through the rolls 48 of the heated roller assembly 50 which dry the fibers before being taken up on the fiber spool device 52. The fibers F undergo stretching in the glycerol bath at a draw down ratio of up to 2.3.

Thus, all together, the fibers are subjected to a draw down ratio of as much as 55 so that the precursor fibers may be drawn down to a diameter of between about 4 and about 10 microns in diameter so as to have a tensile strength of between about 500 and about 1100 MPa and an elastic modulus between about 13 and about 18 GPa. In one embodiment the PAN-based precursor fibers F have a diameter of between about 5 and about 6 micron, a tensile strength of between about 800 and about 1100 MPa and an elastic modulus of between about 15 and about 18 GPa. In one useful embodiment the fibers F undergo a total draw down ratio of between about 15 and about 55. In another useful embodiment the PAN-based fibers F undergo a total draw down ratio of between about 40 and about 55.

As should be appreciated, the hybrid dry-jet gel spinning method described herein produces precursor fibers with relatively small diameter (approximately 5 microns) and high tensile strength (approximately 1,000 MPa). In fact, the present method allows one to produce PAN-based precursor fibers of a given tensile strength using a starting polymer material of lower molecular weight than necessary when producing an equivalent tensile strength fiber using the method disclosed in the Kobashi et al. '027 patent. As higher molecular weight polymer material is more difficult and expensive to manufacture this represents a substantial advantage for the present method. Further, by utilizing a spinning solution with 6 wt % polymer concentration and a viscosity of between about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) at room temperature (between about 20° C. and about 26° C.), it is not necessary to heat the spinning solution 14, the reservoir 12, or the spinnerette 16. In contrast, prior art methods such as disclosed in Kobashi et al. require heating the spinning solution equipment to 80° C. to lower the solution viscosity and produce a spinnable solution. This heating requirement increases production cost.

Further, it is hypothesized that the use of a low solids content spinning dope containing a high molecular weight polymer aids in the drawing a filaments to small diameters, which further minimizes the production of voids within the extruded fibers F. This improves the density and tensile strength of the fibers. In contrast, the relatively large fiber diameter of prior art approaches such as those disclosed in the Kobashi et al. reference leads to the possibility of a higher void content in the extruded fibers, thereby reducing the density and tensile strength of the fibers and increasing the probability of fiber tensile failure. Accordingly, fibers produced by the prior art Kobashi method are not suitable for draw down at higher ratios as they are more prone to tensile failure. Of course, fibers produced by the present method that are characterized by higher tensile strengths are less likely to fail during draw down and production losses and waste are advantageously reduced.

It should also be appreciated that the present method utilizes several baths 22, 26, 30, 34, 42, 46 for fiber stretching. By controlling the composition and temperature of the solutions in the baths 22, 36, 30, 34, 42, 46 it is possible to fine tune the stretching within each bath and reliably produce a small diameter high tensile strength fiber F. A key aspect for achieving a total draw down ratio of up to 55 is the stretching of the extruded fiber F through the air gap and in the coagulation bath (a draw down ratio of up to 8.5 is not unusual). It is possible to achieve this relatively high draw down ratio because the spinning solution contains a high molecular weight copolymer (between 1,000,000-2,000,000 daltons) and has a viscosity of between only about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) and more particularly between only about 200 and about 250 Pa-sec (at a shear rate of 1 l/sec) at room temperature. As the spinning solution remains ungelled until contacting the coagulating fluid, it is possible to achieve high draw down ratios within the air gap (up to about 8.5).

FIG. 2 is a plot illustrating viscosity versus shear rate for four different polymer solutions using the present method versus 5 wt % and 15 wt % polymer solutions using the method disclosed in the Kobashi et al. reference. FIG. 3 is a comparison of fiber tensile results demonstrating that the present hybrid method produces superior precursor PAN fiber in terms of diameter and mechanical properties when compared to the Kobashi et al. approach. FIG. 4 illustrates precursor fiber tensile strength results for fiber produced using the current method versus approximate tensile strength of currently commercially produced precursor PAN fiber, according to Matsuhisa et al. U.S. Pat. No. 6,428,892 B2 (Toray Industries, Inc.). As should be appreciated, the PAN precursor fiber prepared utilizing the present method demonstrates superior strength.

The following synthesis and example is presented to further illustrate the present method but it is not to be considered as limited thereto.

Example 1

A proprietary polymer consisting of polyacrylonitrile and a methyl acrylate comonomer (PAN-co-MA) was heated into a 6.5 wt % solution with N,N-dimethylacetamide (DMAc). The obtained room temperature spinning dope was pressurized using pneumatic cylinders to a metering pump and through a series of filters before encountering the breaker plate and spinnerette.

The spinnerette consisted of 333 extrusion holes, each measuring 150 micron in diameter. The spinning dope was extruded through this spinnerette into air, being allowed to pass through about a 5.5 mm air gap before being introduced into a coagulation bath comprising 60 wt % DMAc aqueous solution, controlled at a temperature of 0° C., to produce a coagulated fiber bundle. The fiber tow was drawn through this bath at a draw down ratio (DDR) of 8.6 times, before passing into wash baths 1-5, as detailed in Table 1.

TABLE 1 Bath 0 1 2 3 4 5 6 7 8 9 Composition 60 50 40 30 10 0 0 Glycerol 0 0 (wt % DMAc stretch in DI H₂0) Temperature 0 5 5 5 5 30 86 172 89 90 Total DDR (C.) DDR in Bath 8.6 1.12 1.02 1.01 1.02 1.13 2.27 2.23 0.94 1.00 55 Fiber 9.9 14.5 14.0 13.5 13.0 12.0 5.1 2.3 2.4 2.4 Total Time Residence (sec) Time in Bath 89 (sec)

In bath 6, consisting of hot deionized water, the fiber was stretched about 2.3 times, before passing over heated godet rollers maintained at about 55° C. to dry the fiber before it entered the hot glycerol stretch.

After being dried, the fiber was passed through a hot glycerol stretch, with the glycerol temperature maintained at about 170° C. Here the fiber experienced the final stretch of about 2.2 times before running through two final water wash baths (to remove residual glycerol), and passing again over a set of heated rollers onto a traversing winder. This process resulted in a PAN precursor fiber tow comprising 333 filaments, drawn at a total draw down ratio of 55 times and having an individual fiber diameter of 5.27±0.47 micron (N=50), a tensile strength of 929±117 MPa (N=29), and an elastic modulus of 17.5±1.6 GPa (N=29).

The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled. 

1. A method of spinning a polyacrylonitrile PAN-based precursor fiber, comprising: extruding a spinning solution of polyacrylonitrile copolymer through a multi-filament spinnerette to produce a PAN-based precursor fiber, said polyacrylonitrile-based polymer having an average molecular weight of about 1,000,000 to about 2,000,000 daltons and said solution having a viscosity of between about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) at a dope extrusion temperature of between about 20° C. and about 26° C.; maintaining an air gap between said spinnerette and a coagulation bath of between about 3 and about 10 mm; stretching said PAN-based fiber in said air gap at a draw down ratio of between about 2.5 and about 8.5; maintaining said coagulation bath between about 0° C. and about 5° C.; and further stretching said PAN-based fiber in a series of stretch baths to produce a precursor PAN-based fiber having a diameter of between about 4 and about 10 micron, a tensile strength of between about 500 and about 1100 MPa, and an elastic modulus between about 13 and about 18 GPa.
 2. The method of claim 1 including using a spinning solution including about 6 wt % polyacrylonitrile copolymer and a coagulation bath consisting of 60 wt % solution of solvent to deionized water.
 3. The method of claim 2, including using N,N-dimethylacetamide as said solvent.
 4. The method of claim 1, including subjecting said PAN-based fiber to a total draw down ratio of between about 15 and about
 55. 5. The method of claim 1 including using a polyacrylonitrile-based copolymer having an average molecular weight of between 1,000,000 and 2,000,000 daltons.
 6. The method of claim 1, including maintaining a dope extrusion temperature of between about 22° C. to about 24° C.
 7. The method of claim 1, including extruding a spinning solution having a viscosity of between about 200 and about 250 Pa-sec.
 8. The method of claim 1, including stretching said PAN-based fiber in said air gap at a draw down ratio of between about 4.0 and about 8.5.
 9. The method of claim 1, including maintaining said coagulation bath at a temperature of between about 0° C. and about 3° C.
 10. The method of claim 1, including producing a precursor PAN-based fiber having a diameter of between about 5 and about 6 micron, a tensile strength of between about 800 and about 1100 MPa and an elastic modulus of between about 15 and about 18 GPa.
 11. The method of claim 4, including subjecting said PAN-based fiber to a total draw down ratio of between about 40 and about
 55. 12. A composition of matter, comprising a PAN-based precursor fiber having a diameter of between about 4 and about 10 microns, a tensile strength of between about 500 and about 1,100 MPa and an elastic modulus of between about 13 and about 18 GPa.
 13. The composition of matter of claim 12 wherein said PAN-based precursor fiber has a diameter of between 5 and 6 microns.
 14. The composition of matter of claim 12 wherein said PAN-based precursor fiber has a tensile strength of between 800 and 1,100 MPa.
 15. The composition of matter of claim 12 wherein said PAN-based precursor fiber has an elastic modulus of between 15 and 18 GPa.
 16. The composition of matter of claim 12 wherein said PAN-based precursor fiber has a diameter of between 5 and 6 microns, a tensile strength of between 800 and 1,100 MPa and an elastic modulus of between 15 and 18 GPa.
 17. The composition of matter of claim 15, wherein said PAN-based precursor fiber is in an unwindable, multifilament, continuous tow.
 18. The composition of matter of claim 12, wherein said PAN-based precursor fiber is in an unwindable, multifilament, continuous tow.
 19. The composition of matter of claim 12, made by extruding a spinning solution of polyacrylonitrile copolymer through a multi-filament spinnerette to produce a PAN-based precursor fiber, said polyacrylonitrile-based polymer having an average molecular weight of about 1,000,000 to about 2,000,000 daltons and said solution having a viscosity of between about 100 and about 300 Pa-sec (at a shear rate of 1 l/sec) at a dope extrusion temperature of between about 20° C. and about 26° C.; maintaining an air gap between said spinnerette and a coagulation bath of between about 3 and about 10 mm; stretching said PAN-based fiber in said air gap at a draw down ratio of between about 2.5 and about 8.5; maintaining said coagulation bath between about 0° C. and about 5° C.; and further stretching said PAN-based fiber in a series of stretch baths to produce a precursor PAN-based fiber having a diameter of between about 4 and about 10 micron, a tensile strength of between about 500 and about 1100 MPa, and an elastic modulus between about 13 and about 18 GPa. 